Gastrointestinal Physiology

Chapter 7 Gastrointestinal Physiology



I. Structure and Function of the Gastrointestinal Tract
A. Functional anatomy
1. Overview
• The gastrointestinal (GI) tract is essentially a hollow digestive tube that extends from the mouth to the anus.

• Secretions from accessory digestive structures such as the salivary glands, pancreas, and liver empty into this tube and are essential for efficient digestion and absorption.

• The digestive tract can be subdivided into three sections based on embryologic origin and vascular supply (Fig. 7-1).

• The foregut extends from the esophagus to the second part of the duodenum and is supplied by the celiac artery.

• The midgut extends from the second part of the duodenum to the splenic flexure of the colon and is supplied by the superior mesenteric artery.

• The hindgut extends from the splenic flexure of the colon to the anus and is supplied by the inferior mesenteric artery.
image

7-1 Splanchnic circulation, which is derived entirely from the celiac artery, the superior mesenteric artery, and the inferior mesenteric artery.



Vascular supply of intestinal tract: foregut—celiac artery; midgut—SMA; hindgut—IMA


• Venous return of all three arterial beds is through the portal vein into the liver.

Pathology note: The portal vein normally carries nutrient-rich blood from the intestines to the liver, after which the blood is shunted to the inferior vena cava through the hepatic vein. In cirrhosis, a variety of pathophysiologic changes result in elevated portal vein pressures, termed portal hypertension. Because the portal vein has multiple anastomoses with systemic veins, pressures likewise increase in these vessels. These systemic veins may then become abnormally dilated and are at increased risk for rupture. In the anterior abdominal wall, venous dilatation can result in caput medusae, a rather harmless clinical examination finding that nonetheless indicates severe liver disease. In the esophagus, venous dilatation can result in esophageal varices. Rupture of esophageal varices can be rapidly fatal.


2. Layers of the gut wall
• Throughout most of the GI tract, the gut wall is composed of four layers; from inside to outside, these are the mucosa, submucosa, muscularis propria, and serosa (Fig. 7-2).
a. Mucosa
• The mucosa is composed of three distinct layers: the mucosal epithelium, the lamina propria, and the muscularis mucosae.
image

7-2 Two different views of the layers of the gut wall.


(B, From Koeppen BM, Stanton BA: Berne and Levy Physiology, 6th ed. Updated ed. Philadelphia, Mosby, 2010, Fig. 26-2.)



Mucosa: composed of mucosal epithelium, lamina propria, and muscularis mucosae


• The structure of the mucosal epithelium varies depending on its location in the GI tract.
(1) Stratified squamous mucosal epithelium is present in the esophagus.

• In contrast, columnar mucosal epithelium is present in the rest of the GI tract, except for the rectum, where it changes back to squamous at the dentate line.

Epithelial cells: stratified squamous in esophagus, columnar in GI until dentate line, squamous in rectum


• In the stomach, the mucosa is folded into rugae.

• In the small intestine, there are folds of cells in the mucosa (villi) and projections on individual cells (microvilli), which increase the surface area for absorption (Fig. 7-3).

• In the large intestine, there are crypts but no villi.
image

7-3 Arrangement of mucosa of the small intestine. Circular folds (plicae circularis), villi, and microvilli significantly increase the surface area of the mucosa. Surface areas are shown in square meters.



Pathology note: In gastroesophageal reflux disease (GERD), the mucosal epithelium of the esophagus takes on the appearance of the gastric mucosal epithelium—it differentiates from a stratified squamous epithelium into a columnar epithelium. This process, whereby one cell type transforms into another, is termed metaplasia. Columnar metaplasia in the lower esophagus is called Barrett esophagus, which can be detected by endoscopy and substantially increases the risk for development of esophageal adenocarcinoma and stricture from scarring. Patients who develop Barrett esophagus require periodic endoscopic surveillance.



Metaplasia: transformation of one adult cell type into another


• The lamina propria is a thin sheet of connective tissue just outside the mucosa.
(1) It contains capillaries, a central lymph vessel, lymphoid tissue, and glands with ducts that allow for mucus and serous secretions onto the mucosal surface.

• The muscularis mucosa is composed of multiple thin layers of smooth muscle, which separate the lamina propria from the submucosa.
(1) Contraction of these layers helps expel the contents of the glandular crypts to the mucosal surface.

b. Submucosa
• Layer of loose connective tissue that connects the mucosa to the underlying muscularis propria

• Contains blood vessels, lymphatic vessels, and nerves that supply the overlying mucosa

• Site of the submucosal plexus, which innervates the muscularis mucosae (more on this later)

Submucosa: contains blood vessels, lymphatic vessels, and nerves (submucosal plexus), which supply the mucosa


c. Muscularis propria (externa)
• Thick muscular layer composed of inner circular and outer longitudinal muscle layers that extend the entire length of the intestinal tract

• Plays an important role in intestinal motility (e.g., peristalsis)

Anatomy note: In the colon, the outer longitudinal muscle layer is discontinuous and clustered into three distinct strips called the taeniae coli.


d. Serosa
• Outermost cellular membrane present in most of the intestinal tract that is continuous with the peritoneal lining

• Cells secrete a serous fluid that helps reduce friction from muscle movement.

• The serosa is absent in certain retroperitoneal portions of the intestinal tract, such as the esophagus and portions of the colon.

Serosa: absent in certain parts of the intestinal tract, such as the esophagus


(1) These portions instead are bounded by a fibrous covering termed the adventitia.

Clinical note: Absence of the serosa from much of the esophagus may contribute to the tendency for esophageal cancers to spread locally before they are detected, in part explaining the poor prognosis associated with esophageal cancer.


3. Neural regulation of the gastrointestinal tract
• Enteric nervous system
a. Overview
• Contained entirely within the gut wall

• Composed of the submucosal and myenteric plexuses

Enteric nervous system: composed of submucosal and myenteric plexuses and entirely contained within gut wall


• These plexuses regulate entirely different aspects of intestinal activity, as discussed later.

b. Submucosal (Meissner) plexus
• Located between the muscularis mucosa and the muscularis propria

• Gives rise to efferent fibers that synapse directly on mucosal epithelial cells, with the primary goal of stimulating secretions required for digestion

Submucosal plexus: stimulates secretion, promotes digestion



Pathology note: In Hirschsprung disease (aganglionic megacolon), the neural crest cells that form the myenteric plexus fail to migrate to the colon. Newborns with this condition are likely to be severely constipated, and imaging studies may reveal a massively dilated colon proximal to the aganglionic segment.


c. Myenteric (Auerbach) plexus
• Located between the inner circular and the outer longitudinal muscle layer of the muscularis propria

• Primary role is coordination of intestinal motility

• Stimulation of the myenteric plexus increases intestinal motility mainly by stimulating peristalsis and also by inhibiting contraction of sphincter muscles throughout the intestinal tract.

Myenteric plexus: promotes motility of intestinal tract


• Extrinsic regulation: autonomic nervous system (Fig. 7-4)
a. The parasympathetic nervous system (PNS) generally promotes digestion and absorption by stimulating GI secretions and peristalsis while inhibiting sphincter muscle contraction.
image

7-4 Innervation of the gastrointestinal tract by the autonomic nervous system. The myenteric plexus synapses mainly on the inner circular and outer longitudinal muscles, whereas the submucosal plexus synapses mainly on the muscularis mucosae and epithelial cells of the mucosa.


(From Damjanov I: Pathophysiology. Philadelphia, Saunders, 2008, Fig. 7-5.)



PNS: promotes digestion and absorption by stimulating secretions and intestinal motility


b. In contrast, the sympathetic nervous system (SNS) generally inhibits digestion and absorption, stimulates sphincter muscle contraction, and causes vasoconstriction in the splanchnic circulation (Table 7-1).

TABLE 7-1 Effect of the Autonomic Nervous System on the Gastrointestinal Tract























Effect Parasympathetic Sympathetic
Motility + −
Sphincter tone − +
Secretion + −
Vasoconstriction No effect +


SNS: inhibits digestion and absorption in part through vasoconstriction of the splanchnic circulation



Clinical note: The PNS stimulates intestinal motility by releasing acetylcholine onto neurons of the myenteric plexus. Therefore, cholinergic drugs should never be given to a patient if an intestinal obstruction is suspected. The resulting increase in pressure could rupture a viscus, resulting in potentially lethal peritonitis.


• Anatomy of reflex loops
a. Local reflexes, such as the gastrocolic reflex, involve afferent and efferent arcs that are contained entirely within the enteric nervous system.

Gastrocolic reflex: gastric distension promotes bowel movement


b. Vagovagal reflexes, such as receptive relaxation of the stomach in response to swallowing of food, involve afferent fibers from the gut that travel through the vagus nerve to the brainstem and then back to the gut through the vagus nerve.

Vagovagal reflexes: afferents from stretch receptors, chemoreceptors, and osmoreceptors in the gut travel to and from the central nervous system through the vagus nerve


c. Afferent fibers from the gut may travel to the spinal cord (or sympathetic ganglia) and then back to the gut.

d. Some of the afferent fibers that travel to the spinal cord synapse, directly or indirectly, on lower-order neurons of the anterolateral system and send pain signals to the brain.
• These afferent fibers that sense pain typically travel to the spinal cord together with the nerves of the SNS.

B. Gastrointestinal functions
1. Motility
• Electrical basis for intestinal motility: slow waves
a. Similar to cardiac nodal cells, intestinal smooth muscle cells (SMCs) have a constantly changing resting membrane potential.

Intestinal SMCs: resting membrane potential unstable and continually depolarizing


b. Rather than constantly generating action potentials, intestinal smooth muscle cells are subject to undulating oscillations in resting membrane potential.

c. These slow waves have a resting membrane potential that varies between approximately −60 and −30 mV (Fig. 7-5).

d. In the absence of spike potentials, the slow waves are unable to elicit smooth muscle contractions, except in the stomach.
• However, if the peak of the slow wave reaches the threshold potential, an action (spike) potential may be initiated, which then stimulates smooth muscle contraction.

e. This rhythmic contraction results in the intermittent propulsion of intestinal contents toward the anus.
image

7-5 Slow waves of the enteric nervous system. The tension occurs slightly after the action (spike) potentials, and the magnitude of the tension depends on the frequency of the spike potentials.



Slow waves → threshold potential reached → action potential generated → smooth muscle contraction → propulsion of intestinal contents toward anus



Pharmacology note: Metoclopramide is a D2 receptor antagonist and 5-HT3 antagonist that is a prokinetic agent useful in the treatment of nausea and vomiting, particularly in patients who have delayed gastric emptying (gastroparesis).


Clinical note: Patients with a long history of poorly controlled diabetes mellitus can sometimes develop severe gastric motility dysfunction, termed gastroparesis. In diabetes, gastroparesis can occur as a result of damage to the autonomic nerves supplying the stomach. These patients may suffer from intractable nausea and vomiting because of the failure of the stomach to empty after a meal. In such patients, promotility agents such as metoclopramide can provide substantial symptomatic relief. A more aggressive option is to surgically implant a gastric pacemaker, although this is rarely done.


• Types of contractions
a. Peristalsis
• Distension of the gut wall by a food bolus triggers reflexive contractions of smooth muscle (mainly the inner circular and outer longitudinal muscle layers) that push the food bolus forward along the intestinal tract.

• This forward propulsion requires smooth muscle contraction just proximal to the food bolus and simultaneous relaxation just distal to the food bolus (Fig. 7-6A).

• The myenteric plexus is almost entirely responsible for coordination of peristalsis.

• In its absence, peristaltic contractions may be severely impaired or even absent.
image

7-6 Peristalsis (A) and segmentation (B).



Peristalsis: dependent on functional myenteric plexus



Pathology note: In a condition called achalasia, esophageal peristalsis is severely compromised because of damage to the myenteric plexus (see later discussion).


b. Segmentation

Segmentation: simultaneous contractions proximal and distal to food bolus; promotes digestion; no forward propulsion of food bolus


• The primary function of segmentation is to assist digestion by promoting mixing of the intestinal contents (e.g., food, digestive enzymes, bile salts).

• This is achieved by simultaneous contractions both proximal and distal to the food bolus (Fig. 7-6B).

• In contrast to peristalsis, segmentation does not result in forward propulsion of the food bolus.

c. Tonic contractions

Tonic contractions of sphincter muscles prevent premature analward passage of intestinal contents.


• The tonic contractions of sphincter muscles throughout the intestinal tract separates different segments of the tract and prevents premature passage of intestinal contents into the next segment.

2. Digestion
• Digestion entails the enzymatic hydrolysis of macromolecules (fats, carbohydrates, and proteins) into smaller compounds.

Digestion: enzymatic hydrolysis of macromolecules into absorbable smaller compounds


• These can then be absorbed across the intestinal epithelial barrier.

3. Absorption
• Absorption involves the transport of luminal substances across the mucosal barrier.

• Absorption is facilitated by the large surface area of the mucosal epithelium, particularly in the small intestine.

Efficient absorption: dependent on large surface area of mucosal epithelium


II. Salivation and Mastication
A. Salivation
1. Composition and functions of saliva
• Salivation plays several important roles in facilitating digestion in addition to its vital role in maintaining oral health (Table 7-2).

TABLE 7-2 Composition and Function of Saliva
























Component Primary Function
Potassium bicarbonate Neutralizes bacterial acid, preventing digestion of tooth enamel and dentine (prevents cavities)
Lingual lipase Initiates lipid digestion
Salivary amylase Initiates carbohydrate digestion
Mucins Lubricate food bolus, are primary determinant of viscosity
Lysozyme Initiates bacterial lysis
Immunoglobulins Offer immune protection


Clinical note: Sjögren syndrome is an autoimmune disorder characterized by lymphocytic infiltration of exocrine glands, mainly affecting the salivary and lacrimal glands. It is relatively common in elderly people (3% to 5% of those >60 years of age) and is characterized by dry mouth (xerostomia) and dry eyes (keratoconjunctivitis sicca). Low levels of saliva may cause dysphagia (difficulty swallowing) and increased dental caries; a deficiency in tear production may cause corneal ulceration and scarring. Pilocarpine, a muscarinic receptor agonist, is effective in increasing salivary production, and artificial tears can be used for treating dry eyes.


2. Mechanism of saliva production
• Secretions from the salivary acinus are very similar in tonicity to plasma (i.e., they are isotonic).

Saliva: usually hypotonic relative to plasma when secreted


• However, as these secretions move along the salivary duct, they are constantly modified.

• The salivary ducts are relatively impermeable to water entry, and sodium is continually reabsorbed.

• Therefore, saliva is usually hypotonic relative to plasma by the time it is secreted (Fig. 7-7).

3. Types of salivary glands
• There are two types of salivary glands: serous and mixed.

• Serous glands (e.g., parotid), which are primarily composed of serous cells, secrete a nonviscous saliva containing water, electrolytes, and enzymes.

• Mixed glands (submandibular, sublingual), which are composed of serous and mucous cells (Fig. 7-8), secrete a viscous saliva rich in mucin glycoproteins.
image

7-7 Saliva production.


image

7-8 Structure of a mixed salivary gland, showing both serous and mucous cells.


(From Koeppen BM, Stanton BA: Berne and Levy Physiology, 6th ed. Updated ed. Philadelphia, Mosby, 2010, Fig. 27-1.)



Salivary glands: two types—serous and mixed


4. Regulation of salivation
• Salivation is primarily controlled by the autonomic nervous system (ANS).

• Both branches of the ANS stimulate salivation, but the PNS does so much more strongly than the SNS.

Pharmacology note: The muscarinic acetylcholine receptor mediates the effects of the PNS on the salivary glands. Blockade of this receptor can substantially decrease salivary secretions. This effect is associated with several classes of drugs, most notably antimuscarinic drugs (e.g., atropine, ipratropium), but also with drugs that have anticholinergic side effects, especially the antipsychotics and tricyclic antidepressants.

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Jul 4, 2016 | Posted by in PHYSIOLOGY | Comments Off on Gastrointestinal Physiology

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